Field of the Invention
The present invention relates to materials that are useful for the containment of corrosive liquids, and more particularly to a carbon-permeated tantalum substrate and a method for its preparation.
Description of Related Art
Containment of corrosive liquids such as liquid metals and molten salts presents a challenge for material scientists. A variety of metallic and ceramic materials have been used conventionally for containment of corrosive materials like actinide metals. For example, U.S. Pat. No. 2,890,110 discloses crucible liners made of magnesium oxide or calcium oxide. U.S. Pat. No. 4,459,153 also uses magnesia crucibles. U.S. Pat. No. 3,328,017 discusses refractory crucibles composed of magnesium oxide, calcium fluoride, calcium oxide, or a mixture of CaO and CaF.sub.2. U.S. Pat. No. 2,894,832 uses a beryllium oxide crucible. U.S. Pat. No. 3,660,075 discloses graphite crucibles coated with niobium carbide or yttrium oxide.
Crucible materials have also included pure tantalum and carburized tantalum having surface layers of tantalum carbide (TaC and Ta.sub.2 C). In particular, U.S. Pat. No. 3,804,939 teaches the use of a tantalum crucible. U.S. Pat. No. 2,908,563 discloses crucibles of graphite and tantalum. U.S. Pat. No. 3,715,204 discloses a crucible made of tantalum and a method for forming hydrides at the interface of the crucible and the product to dislodge the product material.
Tantalum crucibles have several disadvantages though, particularly in containing liquid actinide metals undergoing processing. The molten metals wet the surfaces of the crucible, which leads to chemical and mechanical corrosion of the crucible. The corrosive liquid adheres to the crucible surfaces, attacks the grain boundaries of the crucible material, penetrates along the grain boundaries, and eventually detaches grains of crucible material that can dissolve in and contaminate the liquid. This corrosion causes the crucible to become brittle and eventually to break. The wetting of the crucible by the liquid metal also hinders the removal of the cooled product.
Because of this wetting problem, tantalum containers are often carburized to form more resistant tantalum carbide surface layers. These surface coatings do not remain bonded to the substrate, however, but are stressed during cooling of the melt. A cooled, solidified material like plutonium, for example, has a thermal expansion coefficient quite different from the container material, which causes the layers of tantalum carbide to fracture and rip off during cooling and removal of the solid.
The corrosion and delamination of the tantalum containers prevent their being used for long periods of time or reused over several thermal cycles. Continual replacement of tantalum containers is expensive and may be inefficient. Therefore, a container material is needed that is wettable by corrosive liquids, heat- and corrosion-resistant, and reusable over at least several processing cycles. The materials should have low solubility in the corrosive liquids, be readily fabricable into containers, and lack the weak, vulnerable coatings that fracture during use.